1. Field of the Invention
This invention relates to reinforcing members suitable for use in rubber composite structures and more particularly to a steel ribbon obtained by mechanically deforming round steel stock and then heat treating the stock.
2. Description of the Prior Art
The use of steel in various geometric configurations for reinforcing rubber articles is well known. A generally accepted form of reinforcement is to use steel filaments twisted together to form a strand. U.S. Pat. No. 3,762,145 discloses this well known method for reinforcing rubber articles and particularly pneumatic tires. Another technique for reinforcing pneumatic tires is to use reinforcing elements in the form of thin flat strips. Such elements are disclosed in recently issued U.S. Pat. Nos. 3,667,529; 3,794,097 and others dating back to the nineteenth century.
In copending patent application, Ser. No. 551,224, filed on Feb. 19, 1975 by John M. Chamberlin and assigned to the assignee of this invention, there is disclosed an improved steel reinforcing member. This steel member is in the form of a ribbon characterized by a tensile strength greater than 200 KSI and a microstructure of tempered martensite, bainite or mixtures thereof. This product was obtained by slitting steel coil stock and then continuously heat treating the stock so as to obtain the desired microstructure. It was determined that optimum physical properties in this product were obtained by virtue of the heat treated microstructure.
As an alternative to the approach of obtaining ribbon by slitting coil stock the feasibility of mechanically working high carbon wire into the desired rectangular cross-section was investigated. The starting stock would have a pearlitic microstructure and it was theorized that mechanically changing the cross-section from round to rectangular would yield at least comparable results to the heat treated slit ribbon product. Another significant property desired in a reinforcing member, particularly for pneumatic tires, is that of resistance to fatigue failure. Heat treated slit ribbon was further characterized by a high fatigue endurance limit (FEL). Since the starting stock for mechanical working would be pearlitic in nature it was assumed that a high FEL would be obtained from the round stock because it is well known in the art that pearlitic steels are tougher than martensitic or bainitic steels.
Pearlitic wire was mechanically converted into ribbon by passing it through a series of rolls. It was anticipated that flattening such wire might generate a series of longitudinal edge cracks and these cracks would seriously hamper the use of this product as a reinforcing member. An evaluation of the flattened ribbon showed that edge cracking did not materialize into a problem. Tensile properties of the ribbon were found to be satisfactory, however, the FEL was not comparable to that obtained in heat treated ribbon. The FEL was unexpectedly low when one considers that the starting material was pearlitic and such a deterioration in fatigue was not anticipated. As a matter of fact fatigue resistance was poorer in the rolled and flattened product than in the starting wire stock. For use as a reinforcing member in a rubber composite such as a pneumatic tire the FEL of rolled ribbon was not satisfactory when compared to the FEL exhibited by heat treated ribbon.
The flattened ribbon was obtained by a single pass through a series of rolls wherein the cross-section of the wire was changed from round to rectangular. To improve the FEL various modifications were made in the method of converting the round cross-section. Different rolling practices were employed and no matter what method was employed FEL could not be appreciably improved. The FEL could not be made comparable to that of heat treated ribbon. Finally it was decided to heat treat the rolled product in an attempt to alleviate this serious restriction on the product's applicability. An immediate and unexpected result of heat treatment was the attainment of properties comparable to the heat treated slit ribbon and of particular importance was the marked improvement in FEL.